This invention relates to the supply of electrical power and has particular relationship to the supply of power by a nuclear reactor. This invention has unique applicability to nuclear-reactor power-supply apparatus in which the nuclear reactor is of the pressurized-water (PWR) type having a primary coolant loop coupled in heat-exchange relationship with a secondary steam-generator loop. This invention also applies to boiling-water reactors (BWR) and to the extent that it may be so applied, such application is within the intended scope thereof.
As the load level of a nuclear reactor is changed, it is necessary that the flow of feedwater to the steam generators be changed. Typically, the feedwater flow to each steam generator is controlled by valves; a main valve in a main line and a by-pass valve in an auxiliary branch line which by-passes the main valve. At normal energy levels the control is effected by the main valve. The by-pass valve may be fully or partly opened or closed. At low load levels, predominately during start up, for example, 15% of nominal rated capacity, the main valve is closed and the control is effected by the by-pass valve.
In accordance with the teachings of the prior art, the feedwater control is carried out, both at high and low levels, by the combination of three components: generator water level, steam flow and feedwater flow.
Measured water level is compared against demanded or preset level and put through a proportional-plus-integral (PI) controller which functions to eliminate steady-state level errors. In addition there is a feedwater flow-steam flow mismatch channel which serves to anticipate an incipient level error. The summed level and flow mismatch signals then go through another proportional plus integral controller which eliminates steady state errors in feedwater flow.
The prior-art control functions satisfactorily at moderate or higher load levels but not at low load levels, for example, below 15% of nominal rated load. In fact because of this deficiency, automatic control of feedwater has not been feasible. Unless the feedwater is properly controlled at low levels, there is "chugging" during start up. This condition may be described as violent hunting induced by the flow of excessive cold feedwater into the generator while it is attempting to generate steam responsive to the command to start up.
It is an object of this invention to overcome the difficulties and disadvantages of the prior art and to provide nuclear-power generating apparatus in whose operation, particularly at low load levels, the feedwater flow shall be reliably, effectively and accurately controlled. It is also an object of this invention to provide such apparatus which shall lend itself to automatic control of feedwater flow at low load levels, typically between 0 and about 15% power operation where the by-pass feedwater line has a maximum capacity of about 20% of the nominal main feedwater flow.
Also, it is an object of this invention to provide an effective anticipatory signal for controlling the feedwater flow at low levels.